According to the text, what is the behavior of natural resource prices in relation to global prices?

The prices of natural resources have remained constant over the past century.
The prices of natural resources have increased significantly over the past century.
The prices of natural resources have decreased or remained constant in relation to global prices over the past century.
a) Only I is correct.
b) Only II is correct.
c) Only III is correct.
d) I and II are correct.
e) II and III are correct.